Coating of paper



Patented Sept. 4, 1951 COATING OF PAPER John Alexander Montgomerie, Cambuslang, Scotland, assignor to Chemical Company, Dublin, Eire Manufacturing N Drawing. Application February 14, 1948, Serial No. 8,307

11 Claims. (01. 106-144) This invention relates to the coating of paper, to obtain, after calendering, a surface finish comparable with or superior to that of the best quality printing paper now available.

In order to obtain highly finished paper suitable for printing, it is necessary to provide the paper with a surface which will take printed impressions without spreading or blurring. To provide such a surface it has been proposed to treat the paper before calendering with a coating containing casein or a caseinate. While casein is of unique value in compositions for the coating of paper to be used for printing, it is a relatively expensive material and adds considerably to the cost of the coating composition.

The present invention contemplates a new and improved casein-containing coating composition in the form of a dispersion of a vegetable resin in an aqueous casein solution, a dry powder for mixing with water to form such a dispersion, and a paper product produced by coating the surface of paper with the improved casein-containing composition.

Like casein-containing paper-coating compositions heretofore used, the compositions of the present invention depend upon the presence of casein to provide a surface suitable for high quality printing. However, the amount of casein which is used is very considerably less than that heretofore used to obtain comparable results, the balance being replaced by a relatively cheap vegetable resin which is dispersible in an aqueous alkaline casein solution. When such a resin is added to an aqueous alkaline casein solution within certain predetermined amounts, a dispersion is formed, which, due to the stabilizing effect of the casein, is exceedingly stable, will not separate into its component parts when applied to paper and will leave on the paper a coating having printing properties equal to or superior to a paper coated with the usual casein coating compositions which do not contain such a resin.

When compositions of the present invention are applied to the surface of paper the aqueous phase thereof wets the paper, and, as the water evaporates from the composition or is absorbed by the underlying paper, the casein still being in the continuous phase firmly attaches itself to the surface of the paper. As the composition does not separate into its component parts on drying of deposited coatings thereof, the casein remains as a continuous film surrounding each of the dispersed resin particles and any dispersed particles of pigment or filler which usually is included in the coating composition. The resin not only serves to lower the cost of the coating compositions by replacing a substantial amount of the relatively more expensive casein, but also functions as a plasticizer for the casein so that a better surface for printing is obtained.

In the usual casein-containing compositions for coating paper, the casein may be present to the extent of from about 5 per cent to 25 per cent on the weight of the aqueous vehicle, depending upon the character of the coating Which is desired and upon the amount of filler or pigment such as china clay, or blanc fixe incorporated therein. In the coating compositions of the present invention, a part of the casein which normally is used is replaced by .a vegetable resin, such as rosin, or colophony, or mixtures of vegetable resins. Such resins when dispersed in an aqueous alkaline casein solution produce compositions which form coatings equal to or superior to coatings deposited from casein solutions or dispersions not containing such resins. The fact that such coatings may be superior to a coating not containing such resins is due, I believe, to the plasticizing effect which such resins have on the casein.

The combined amount of casein and resin material, and the amount of pigment or filler, such as china clay, blanc fixe, etc., which are used in compositions of the present invention is not critical and may vary over a wide range. The combined amount of casein and resin may be the same as the amount of casein now used in similar compositions. Likewise, the ratio of the combined amount of casein and resin to filler or pigment, when such pigments or fillers are included in the composition, may be the same as the ratio of casein to filler or pigment heretofore used, the relative amounts of combined casein and resin, and pigment used being determined by the particular paper being coated and the nature of the coating desired, or both. For example, the combined amount of casein and resin may be present in the compositions in amount from about 3 to 20 per cent, and the filler or pigment in amount from 10 to per cent, although in some instances lesser or greater amounts may be usedv The use of the resin in any amount whatever lowers the cost of the coating composition proportionately, but in order to obtain the desired benefits, considering both the cost of the compositions and the nature of the coatings deposited therefrom, the ratio of plasticizing resin to casein should be between about 1:1 and 1:3, but preferably is maintained between 1:2 and 1:3.

It will be understood that the ratio of resin to- 3 casein within the above limits will be determined by such factors as costs, nature and purposes of the matter to be printed on the coated paper, and the degree of whiteness which the coating is to impart to the paper. In order to obtain a reasonably cheap composition which will give a satisfactory coating for many purposes, the ratio of resin material to casein may be as high as 1:1. On the other hand, to produce a composition which will deposit a coating on paper which definitely is equal or superior in behaviour towards printing inks to that given by the usual casein-pigment compositions, the ratio of resin material to casein should be kept between about 1:2 and 1:3. Regard also should be given in preparing the compositions to the desired colour to be imparted to the paper and where a good white colour is desired, the ratio of resin material to casein should not be higher than about 1:15.

In preparing compositions of the invention using rosin as the plasticizing material, the rosin may be first dispersed in an aqueous alkaline solution and the resulting dispersion admixed with an aqueous alkaline solution of casein. Suitable alkalis for ,making the aqueous alkaline solution are ammonia, caustic soda, or caustic potash. The china clay or other pigment or filler to form a constituent of the composition is then stirred into the mixture until a good dispersion is obtained, having unsaponified rosin and pigment or filler dispersed therein in fine particles.

The rosin may be first heated together with the oil, or some part thereof, to give a homogeneous liquid, which is then admixed with the alkaline aqueous casein solution in the same manner as the rosin by itself. If the rosin is first heated with the oil in this Way, the mixture may be emulsified in an aqueous solution of an alkali with or without the use of an additional emulsifying agent, since the rosin soap which is formed will itself aid emulsification.

The composition prepared as set forth above, with, if desired, a suitable pigment or filler added thereto and uniformly dispersed in the casein solution which forms a continuous phase of the composition, may be used as prepared. Alter nately the composition without addition thereto of any pigment or filler may be spread as a film on a smooth surface, for example, a revolving heated drum, and allowed to dry, after which the dried material is scraped off and brushed through a 100 mesh screen to give a dry powder, which may be transported and stored for subsequent use. This powder disperses readily when stirred into water or dilute aqueous alkaline solution, forming a composition indistinguishable from the original preparation. When the material is to be converted in this way into a dry powder, the pigment is not added prior to the drying thereof; when desired, it may be added to the composition re-established by dispersing the dry powder into water. The pigments or fillers usually are readily available to paper manufacturers, and by omitting the pigments or fillers from the emulsion prior to the drying thereof, the resulting dry powder is much lighter and contains less bulk so that transportation charges are reduced to a minimum.

It is a great advantage of compositions as contemplated by the present invention that they may be dried to form a powder or flake material which can be stored and transported in the solid state, and can be reconverted to form satisfactory coating compositions at will, merely by mixing with water or alkaline solution. The powder or flake material, as a general rule, is quite dry to the touch and does not show any appearance or feel of greasiness or oiliness, due, apparently, to the fact that on drying the casein forms an adsorbed coating on each of the dispersed particles of the resin.

The reconversion is aided if the water which is added to the dry powder or flake material contains a small proportion of alkali such as ammonia or caustic soda.

The invention will be further illustrated by the following examples, but it is to be understood that such further illustration is merely by way of exemplification and the invention is not limited to the use of. the specific amounts of the constituents recited in the several examples.

Example I A casein solution is made up containing the following, by weight:

Parts Casein 40.0

NaOI-I 2.4

Water 223.6

To this solution is added a rosin dispersion made up from the following constituents, in the amounts indicated, by weight:

Parts Rosin 30.0 NaOH (actual) 3.3 Water 100.7

The total of 400 parts thus contains 30 parts rosin and 40 parts casein, a total of '70 parts; the ratio of rosin to casein thus is 3:4, or 1:133 and the rosin constitutes about 43% of the total weight of casein and rosin present.

The mixture may be spread as a film on a smooth surface and allowed to dry. The dried material may be scraped off and brushed through a -mesh screen; the powder so obtained may be stored as required, and may be redispersed by admixture with four times its own weight of water containing 2 to 4 parts of ammonia of 0.88 S. G.

An excellent coating composition is made by mixing'42 parts of the original dispersion or of the dispersion formed by reconverting the dried material, with 100 parts of a slurry made of equal parts by weight of china clay and water.

. Example II A solution of casein was formed of the following constituents, by weight: 1

Parts Casein 112.0 NaOH 646' Water L 626.0.

265 parts of this solution were mixed with 100 parts of a rosin dispersion of the following composition, by weight:

Parts Rosin .60.0 NaOH 6.6 Water 200.0.

The resulting dispersion was dried. The dispersion was re-established by adding parts of a china clay slurry containing equal parts of china clay and water to 42 parts of this dried composition, with suitable agitation.

The ratio of rosin to casein in the resulting composition was about 1 :1.6.

Example III 8.4 parts of the dried composition of Example II were reconstituted to the form of a dispersion by addition thereto, by weight:

Parts Water 54.0 Ammonia (S. G. 0.88) 0.1

The reconstituted dispersion contained substantially the same ratio of solids to water as the emulsion of Example II.

Example IV In this example, the procedure of Example I is followed, but the weight of rosin used is reduced from 30 to 26.7 giving a ratio of rosin to casein of 1:1.5, with the rosin constituting 40% of the total of the rosin and casein present.

In preparing the above emulsions, and in reconverting the dried powder to emulsion form, the necessary heat may be applied either to cause the emulsion to form, or to facilitate its formation.

Compositions prepared as set forth above are very stable due, at least in part, to the powerful stabilizing effect of the casein. When applied to paper the aqueous medium immediately Wets the cellulose fibres of the paper and as it evaporates from the deposited coating of the composition, the casein dissolved therein firmly attaches itself to and makes a firm bond with the paper. Due to the high stability of the composition it does not separate into its component parts as the water gradually evaporates therefrom to form separate layers of resin and casein, but the casein continues to coat the dispersed particles of resin and pigment and remains as an adsorbed film on them and thereby imparts to them the surface characteristics of casein. The resin also acts as a plasticizer for the casein and thereby further improves its characteristics as a paper-coating material. As the film of casein adsorbed by each of the resin and pigment particles imparts to them surface characteristics of the casein itself, the entire coating acts much as would a coating of pure casein and gives the superior surface finish desired on printing papers, while at the same time effecting a substantial saving in the amount of casein used in the coating with a proportionate reduction in the cost of the composition.

The nature and proportion of the resin which is added to the composition of the present invention may be varied within the above stated limits, according to the nature of the surface desired, and, if desired, wetting-out agents may be added.

In all instances the filler which is used in the coating composition should be inert, that is, one which does not react with other constituents of the coating composition.

This application is a continuation-in-part of my abandoned application Serial No. 765,814,

filed August 2, 1947, which in turn was a continuation-in-part of my abandoned application Serial No. 536,614, filed May 20, 1944.

As it is not entirely clear what takes place when casein is added to an alkaline aqueous solution, i. e., whether the casein goes into true solution or forms a colloidal solution, or whether the casein reacts in Whole or in part with the alkali of the solution to form a caseinate which then goes into true solution or forms a colloidal solution, the term solution, as used herein and in the appended claims with respect to casein covers the product resulting from the addition of casein to alkaline aqueous solutions; the term casein covers both casein itself and caseinatesj and the term filler" includes pigments of the kind ordinarily used in paper coating compositions which perform a filling as well as a pigmenting function.

What is claimed is:

1. A paper-coating composition having a continuous phase of an aqueous alkaline solution of casein, and dispersed therein a vegetable resin, at least a part of which is in the form of an alkalimetal resinate, the ratio of the total vegetable resin to the total casein in the composition, by weight, being between 1:1 and 1:3.

2. A paper-coating composition as set forth in claim 1 in which the vegetable resin is rosin.

3. A paper-coating composition as set forth in claim 1 in which the composition is an emulsion and includes mineral oil in the dispersed phase.

4. A paper-coating composition as set forth in claim 1 which also includes an inert filler.

5. A paper-coating composition as set forth in claim 1 in which the ratio of vegetable resin to casein is between 1:2 and 1:3.

6. A dry powder for mixing with water to form a composition for coating paper comprising casein and a vegetable resin, the ratio of the vegetable resin to casein, by weight, being between 1:1 and 1:3, and at least a substantial part of the casein being present as an adsorbed coating on the particles of the vegetable resin.

7. A dry powder for mixing with water as defined in claim 6 in which the ratio of vegetable resin to casein is between 1:2 and 1:3.

8. Paper having a surface finish formed of a coating of the composition of claim 1.

9. Paper having a surface finish formed of a coating of the composition of claim 3.

10. Paper having a surface finish formed of a coating of the composition of claim 4.

11. Paper having a surface finish formed of a coating of the composition of claim 5.

JOHN ALEXANDER MONTGOMERIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,044,056 Burgard June 16, 1936 2,195,600 Reilly Apr. 2, 1940 2,301,298 Light Nov. 10, 1942 2,386,033 Chappell Oct. 2, 1945 2,393,179 Mashburn Jan. 15, 1946 2,401,090 Mashburn May 28, 1946 

1. A PAPER-COATING COMPOSITION HAVING A CONTINUOUS PHASE OF AN AQUEOUS ALKALINE SOLUTION OF CASEIN, AND DISPERSED THEREIN A VEGETABLE RESIN, AT LEAST A PART OF WHICH IS IN THE FORM OF AN ALKALIMETAL RESINATE, THE RATIO OF THE TOTAL VEGETABLE RESIN TO THE TOTAL CASEIN IN THE COMPOSITION, BY WEIGHT, BEING BETWEEN 1:1 AND 1:3. 